This invention relates to a scanning type readout device which converts various images of originals or the like which are hand-written, typed, or printed, into electric signals in a time-sequence. More specifically, the invention relates to a scanning type readout device of small size and low cost which comprises a plurality of photoelectric conduction cells arranged in one line. A plurality of semiconductor switches are connected to the two electrodes of the same cells for scanning the same, a signal detecting load resistance, and a constant voltage d.c. power source, together constitute a solidstate light-receiving element array. An image focusing optical system is employed for projecting the image of an original on the photoelectric conduction cells.
In facsimile telegraphy for transmitting images such as printed letters or drawings, optical readout devices of various types and complicated constructions have been heretofore used.
FIG. 1 illustrates a basic circuit of an optical diode array (PDA) known in the prior art. In this circuit, D.sub.1, D.sub.2, . . . , D.sub.n designate photodiodes, S.sub.1, S.sub.2, . . . , S.sub.n designate FET switches, R a load resistor, E a d.c. power source, G.sub.1, G.sub.2, . . . , G.sub.n gates of the FET switches. The scanning circuit of the gates is composed of shift registers S.sub.R.
The principle of the operation of the photodiode array can be easily understood if an element of the array is closely studied. FIG. 2 is an equivalent circuit of the element, wherein a photodiode D is represented by a parallel circuit of a photoswitch P.sub.D and a sensor capacitance or a p-n junction capacitance C.sub.j. SA and SB designate scanning FET switches, and R designates the outside load resistor.
In a state where SA and SB are closed, a reverse bias voltage E is applied to the photodiode D (P.sub.D and C.sub.j), and an electric charge Q=C.sub.j E is stored at the p-n junction thereof. This electric charge Q is ordinarily termed a saturated electric charge. After the termination of the charging, the switches SA and SB are opened, and immediately thereafter, the irradiation of light is started. Assuming the sensitivity of the photodiode D is IA/lux, the electric charge Q' discharged in the photodiode D at the time a light of an intensity L is irradiated during a time T can be expressed as, EQU Q'=I.multidot.L.multidot.T
The product L.multidot.T at the time Q' is made equal to Q is termed the saturated light exposure. When the switches SA and SB are again closed, an electric charge corresponding to the charge Q' having been discharged within the diode is recharged. This recharging current is detected across the terminals of the load resistor R to be used as an image signal.
In the case where a plurality of photodiodes D are arranged in a series, and the switches SA and SB are opened or closed sequentially, output image signals can be picked up from a load resistor R commonly provided for these photodiodes. This feature is extremely advantageous from the view point of common use of an amplifier circuit.
Since a photodiode array or CCD sensor currently available on the market are made in an integrated circuit condition, a diminishing type optical system must be used for reading-out the original. This inevitably increases the size of the optical system and such a feature is disadvantageous for the provision of an original readout device of a reduced size. The use of a 1:1 image focusing optical system of, for instance, utilizing selfocus lenses, is also not advantageous because it requires a light receiving element array of a width equal to that of the original. Hence if the array and the associated parts are made of separate semiconductor switches available on the market, the readout device wil be excessively costly and too large in size.